The present invention relates generally to aircraft control systems and more particularly to aircraft performance management systems for controlling the aircraft vertical flight profile through the coordinated operation of the engine throttles and pitch attitude to thereby provide the most cost effective operation of the aircraft.
Most modern commercial transport aircraft, many general aviation aircraft and certain military aircraft include automatic flight control systems and/or flight director systems, for controlling the aircraft attitude and flight path as well as automatic throttle control systems for controlling aircraft thrust. For many years these two systems have been operated more or less independently or coordinated only during certain phases of flight, such as for example during take-off and landing operations the primary consideration being aircraft controllability and safety rather than operating costs. However, in view of the more recent substantial increases in fuel costs, aircraft operators are very desirous of increasing fuel efficiencies throughout the entire vertical flight profile of their flight plans by assuring the most cost effective operations possible. Coordination between the automatic throttle controls and the automatic pitch attitude flight controls have been carefully analyzed in attempts to achieve these results. Recently developed systems, resulting in this coordinated automatic thrust/attitude control for aircraft have been termed flight management or performance management systems. In general, such systems compute and control thrust and pitch parameters to achieve minimum fuel consumption with the constraints of scheduled trip time between the start of the take-off run to touchdown at the destination. A particular objective of such systems is to obtain smooth, stable and accurate control when the automatic system commands the aircraft to accelerate, decelerate or change its flight path in the vertical plane. In one such system as disclosed in the present inventor's copending U.S. patent application Ser. No. 332,901, filed Dec. 21, 1981 and entitled "Cruise Speed Control for Aircraft Performance Management System", also assigned to the present assignee, a desired and usually automatically commanded airspeed, usually expressed in terms of Mach number and hereinafter designated as Mach target (M.sub.TGT), is applied to both the automatic throttle control system and to the pitch channel of the automatic pilot and/or flight director system. The M.sub.TGT signal to the auto-throttle is used to compute and establish a corresponding engine pressure ratio (EPR) or engine fan speed (N.sub.1) while the same M.sub.TGT signal to the autopilot is used to pitch the aircraft so as to capture the targeted or commanded Mach speed.
One area of particular concern which prior to such automated systems have been primarily under pilot manual control is the several speed changes which are required by the particular aircraft's characteristics, by an airline's normal operating procedures, by federal regulations and local authorities, at various stages of a flight. Such speed changes occur at the start of second segment climb after take-off, when accelerating to flap retraction speed, when accelerating to 250 KIAS at 3000 feet and when accelerating from the 250 knots requirement below 10,000 feet, flight level 100 (FL100), to optimum climb speed. Other requirements involve particular requirements of Air Traffic Control (ATC). Also, similar deceleration restraints and procedures may be placed on the aircraft during descent from cruise altitude.
Prior to transitioning from the 250 KIAS at FL100 to optimum climb KIAS, for example, the engines are usually set at maximum climb thrust, that is, at the EPR limit values recommended by the aircraft and engine manufacturers. These limit values are variable and are calculated from such existing conditions as inlet air temperature, pressure altitude and the state of various engine bleeds, to provide a corresponding maximum thrust. In general, a desired airspeed may be captured and maintained by controlling pitch attitude. If the aircraft is climbing out at a pitch attitude that will maintain the required airspeed of 250 knots and it is required that the aircraft accelerate to a more economical speed as it passes through 10,000 feet, the performance management system commands (or the pilot selects) an airspeed change from 250 knots to a typical value of 300 knots. If this commanded change is inserted into the pitch controls as a step function, or substantially a step function, the aircraft may nose over excessively and actually dive in its attempt to capture the commanded airspeed. It is desired not be permit such excessive maneuver but rather to proportion acceleration and vertical speed in an adaptive manner to achieve a reasonable rate of climb since low rates of climb for the existing thrust, drag and weight conditions are wasteful of fuel due to the higher fuel consumption of the engines during low altitude operations. Known prior art performance management systems establish some predetermined small climb rate, such as, for example, 500 feet per minute during accelerating climb. This arbitrary climb rate is optimum for only low excess thrust over drag and high aircraft weight and, hence, fuel economy is compromised for other conditions.